The answer makes sense to me. Adding solid leaves the system in equilibrium (i.e. Ksp = [Ca2+]*[CO32-] says the same after the addition of solid), so by the definition of equilibrium:
rate of dissolution = rate of crystalization
Therefore, adding more solid increases both the rates of crystalization and dissolution because of the increased surface area of solid. This makes sense because both rates must increase by the same amout for the system to remain in equilibrium.